As has been pointed out in the aforementioned copending application and as will be apparent from the German patent document (printed application--Auslegeschrift) DE-AS No. 2347 934, it is already known to provide ceramic burner heads with parallel passages through which the fuel and the combustion sustaining air can be passed to the combustion chamber in which the two fluids mix and are ignited to sustain combustion and thus generate industrially or otherwise usable energy. The advantage of a ceramic burner head of the aforedescribed type is that it can also serve as a recuperative heater for the fluids introduced into the combustion chamber (see German patent document--open application--Offenlegungsschrift DE OS No. 06 27 043).
In this system the gas mixture of the fuel gas and the oxidizing agent, prior to entry into the combustion chamber, is contacted with a recuperatively heated surface whose temperature has been raised by contact with or conduction from exhaust gases generated in the combustion chamber and in the course of discharging such gases from the combustion chamber.
In connection with burner heads for the purposes described, mention may also be made of the high-output burner head described in German patent document (printed application--Auslegeschrift) DE-AS No.20 44 813. In this head a plurality of parallel flow passages of the fuel gas and the oxidizing medium are disposed adjacent one another so that the flow passages of the oxidizing medium are surrounded by the flow passages for the fuel.
In, for example, German patent document (Open application--Offenlegungsschrift) DE-OS No. 24 08 542, preheating of the combustion air is described before the combustion air is mixed with the fuel gas in a system in which the combustion air acts as a coolant for metal walls of the combustion chambers. While this system provides an increased efficiency, it is frequently desirable to heat the fuel as well before it is mixed with the oxidizing agent for the reasons stated above.
This effectively represented the state of the art prior to the system developed by some of us and described in the aforementioned copending U.S. application. In that system the ceramic burner head was formed with passages for the separate supply of fuel agents and an oxidizing agent to the combustion chamber via a plurality of mutually parallel and adjacent passages, opening into the combustion chamber for the fuel and the combustion air.
The flow passages had an elongated cross section (preferably rectangular with the length of the cross section many times greater than the width) and the flow cross sections were closed on opposites sides of the ceramic body by cover plates or walls. The flow passages were offset from one another so that a partition was formed in the ceramic burner body between each pair of neighboring passages to enable indirect heat exchange between the oxidation gas or combustion air and the fuel. In the preferred construction of the system, part of the length of each combustion air passage was paralleled by a passage for the exhaust gas (hot combustion gas) so that the combustion air was first heated by recuperative heat exchange and then transferred heat to the fuel by indirect heat exchange through the partitions.
When ceramic burner heads of the earlier designs, and even of the latter application, are provided in industrial furnaces or even as low-power energy procedures for other purposes, one may wish to make use of liquid fuels such as fuel oils.
In such cases it is desirable that the heat transferred from combustion air or oxidation fluid to the fuel be sufficient to vaporize the fuel before it emerges from the mouth of the burner.
However, various problems ensue. With prior art systems, and even the system described in the above identified copending U.S. application it is not possible to always avoid overheating of the fuel and a cracking thereof which produces detrimental residues which can block the flow passages of the burner head or give rise to deposits in the combustion chamber or in the exhaust gas passages.
It may also be mentioned that earlier systems having multiple passages for the fuel and oxidation agent, and even the systems of the copending U.S. application mentioned above, while highly effective for low power systems, may not be suitable for high-power furnace installations because of limited throughput which results from the desire to obtain an efficient mixing of the fluids and heat exchange between them.